1. Field of the Invention
The present invention relates to a method and apparatus for experimentally determining the corrosion rate of a test plate of a given metal by applying alternating current voltages of different frequency between a pair of such plates and obtaining an indication of the corrosion resistance of the metal from the different effects of the corrosion reaction upon the resultant currents between said pair of plates.
2. Description of the Prior Art
It is very important in designing many kinds of apparatus and equipment, as regards selection of materials of construction, projecting needed maintenance for such apparatus and equipment, and preventing accidents in their operation, to know in advance with reasonable certainty the corrosion rate of a given metal when subjected to given environments. Some methods of measuring corrosion rates have been already suggested. Among these are included, for example, a corrosion weight loss method wherein the variation of the weight of a sample metal before and after being subjected to corrosion for a given time is measured, a Tafel external polarization method wherein an electric current is made to flow through a sample, its electrode potential is measured, and the linear relationship between the potential and the logarithm of the current (Tafel relation) can be used to determine the corrosion rate, a polarization resistance method wherein a small current is made to flow through a sample and the linear relationship between the potential and current is utilized, a rectangular wave polarization method wherein a small current pulse is repeatedly generated and the corrosion rate is calculated from the relation of the potential to the current, and an electric resistance method wherein the sample is made in the form of ribbon or wire, and the reduction of the cross-section due to corrosion is measured as an electric resistance variation. However, these methods have various defects. In the corrosion weight loss method, only the average corrosion rate over the period between weight measurements is given, and accurate measurement is difficult where the corrosion rate varies with the lapse of time or a significant change in weight is not obtained within a short time. In the Tafel external polarization method, the current required to satisfy the Tafel relation is so large that the errosion of the surface condition of the sample and corrosion environment will be disturbed by the electrolysis, the Tafel relation is not easy to obtain, and the determined corrosion rate will vary by a factor of at least 100 from the actual rate in most cases. In the polarization resistance and rectangular wave polarization methods, reference electrodes other than the sample plates are required and are difficult to set and maintain and results are subject to large error for corroding environments (such as, for example, in industrial water) which are high in solution resistance. In the electric resistance method, there is a danger of exaggerating the extent of localized corrosion of the sample.